In automatic control systems, feedback signals representing measurements of parameters under control are compared against the desired values set by the operator. A difference or an error signal is derived from the comparison and the parameter is further adjusted to reduce such error. The parameter is set as desired when the error is reduced substantially to zero. For high performance control, it is essential that the parameter sensors accurately and precisely measure the parameters under control. For motor control systems, parameters such as velocity and position must be accurately and precisely measured.
In prior motor feedback control systems, devices such as resolvers and tachometers have commonly been used to measure motor position and velocity.
Motor velocity measurement by an analog tachometer suffers from the inherent problems of an analog system; i.e., problems of offset, scaling inaccuracies, and temperature drift. A digital velocity value can be obtained by using an analog to digital converter, but this method does not eliminate the inherent problems associated with analog devices. Furthermore, a sufficiently precise analog to digital converter requires a large number of bits of resolution and would become too expensive for most systems.
Motor angular positional information is often derived by means of a resolver and a resolver to digital converter. The angular position value is measured and then quantized into a digital feedback value. The precision of such digital feedback system is dependent on the resolution of the quantization boundary. The resolution of the digital data from the quantization is inversely proportional to the motor speed being quantized; that is, as the motor speed increases, the resolution or the quantization boundary in number of bits derived will necessarily decrease. For example, when a motor increases its speed from 300 rpm to 1500 rpm, a digital resolver which produces a 16 bit quantization at 300 rpm will typically have a reduction in resolution down to a quantization boundary of 14 bits.
For added precision, both the digital and analog feedback values can be combined in one system. In such a system the resolver will output both digital and analog feedback values. The digital data from the resolver represents the quantized positional value and is used in the digital feedback loop. The analog feedback output represents the error signal between the quantized positional value and the actual motor position. In such a system, the motor is adjusted in a digital feedback loop according to the digital feedback value. The system then switches over to the analog feedback loop for fine adjustment according to the analog feedback signal. The latter analog feedback loop is known to one skilled in the art as the analog lock feedback loop.
Velocity feedback data for motor speed control can be similarly obtained from positional feedback information by differentiating the feedback values from the resolver.